"Cannot insert explicit value for identity column in table 'x' when IDENTITY_INSERT is set to OFF" - inserting record with nested custom object

Question

I get the error "Cannot insert explicit value for identity column in table 'UserPermission' when IDENTITY_INSERT is set to OFF" trying to insert a record as follows:

dbContext.User.Add(someUser);
dbContext.SaveChanges();

That being said, the User file has the custom class UserPermission as one of its parameters, and someUser's UserPermission is not null and has a set ID parameter. Why does this happen and is it possible to avoid getting this error without having to explicitly add a UserPermissionID foreign key parameter in my User model and setting the UserPermission parameter to null?

Thanks in advance.